Sodium carbonate recovery from waste streams and impounded sodium carbonate decahydrate deposits

ABSTRACT

A process is described for recovering sodium carbonate or other sodium-based chemicals from sodium-bearing streams, including in particular mine water, evaporative pond water and sodium carbonate decahydrate deposits, recycle and purge streams, and other waste streams. In the process selected sodium bicarbonate-bearing streams are decarbonized to reduce the sodium bicarbonate concentration in a combination with other sodium-bearing streams, resulting in a liquor suitable as feed to a sodium carbonate decahydrate or sodium carbonate monohydrate process. The sodium bicarbonate concentration can be reduced using any number of known processes such as reacting said sodium bicarbonate with a neutralizing agent such as calcium oxide, calcium hydroxide, sodium hydroxide, or other alkali. Sodium bicarbonate can also be stripped using steam or air. The sodium bicarbonate reduced stream is combined with other sodium-bearing streams where the concentration is adjusted to form a liquor suitable to feed a sodium decahydrate or sodium carbonate monohydrate evaporation/crystallization step. Alternatively, the decarbonized stream can be concentrated using sodium carbonate decahydrate crystals formed from said sodium carbonate decahydrate process. Additionally, waste streams dilute in sodium carbonate concentration can be heated, especially with waste process heat, and recycled to existing sodium carbonate decahydrate deposits in evaporation ponds prior to combining said stream with other waste streams, purge streams, recycle streams, or sodium decahydrate crystals with the intention of recovering sodium carbonate from such streams and deposits and further processing the resulting liquor through an evaporation/crystallization step whereby various selected sodium carbonate salts are produced. The combination of the various sodium-bearing streams is decarbonized to below 3.5% sodium bicarbonate when fed to a sodium decahydrate process and to below 1% sodium bicarbonate when fed to a sodium carbonate monhydrate process. The feed streams are adjusted in sodium carbonate concentration by higher concentrated sodium carbonate-bearing streams or by addition of sodium carbonate decahydrate produced from said streams or recovered form evaporation pond deposits, are then processed to produce sodium carbonate decahydrate or sodium carbonate monohydrate or further processed to form other sodium carbonate salts.

[0001] This invention relates to a process for recovering sodiumcarbonate or other sodium based chemicals from sodium carbonate-bearingstreams, especially mine water, evaporative pond water, recycle andpurge streams, and other waste streams obtained in the course ofprocessing trona.

BACKGROUND OF THE INVENTION

[0002] Processes favored in the United States for the production of sodaash are the sodium carbonate sesquicarbonate process and the sodiumcarbonate monohydrate process. Both processes purify crude trona toproduce refined soda ash. Almost all of the soda ash produced in theUnited States is obtained from a vast deposit of crude, mineral trona(Na₂CO₃.NaHCO₃.2H₂O) located in Green River, Wyo. The trona deposit ismade up of several trona beds about 800 to 3000 feet underground. Thesetrona beds are separated by layers of shale that generally overlap eachother. The quality of the trona varies depending upon its location inthe deposit. Crude trona consists primarily of about 87-88% of sodiumsesquicarbonate (Na₂CO₃.NaHCO₃.2H₂O) and about 11-13% insoluble claysand shales, and in lesser amounts, sodium chloride (NaCl), sodiumsulfate (Na₂SO₄), and organic matter. The amount of impurities presentis sufficiently large so that this crude trona must be purified toremove or reduce the impurities before the soda ash or other sodiumcarbonate salts derived from the trona can be produced and sold forcommercial use.

[0003] Sesquicarbonate

[0004] The Sesquicarbonate processing steps involve: dissolving thecrude mined trona in a cycling, hot mother liquor containing excessnormal carbonate over bicarbonate in order to dissolve the tronacongruently, clarifying the insoluble muds from the solution, filteringthe solution, passing the filtrate to a series of vacuum crystallizerswhere water is evaporated and sodium sesquicarbonate crystals form andare separated from the mother liquor, recycling the mother liquor todissolve more crude trona; and calcining the sesquicarbonate crystals tosoda ash.

[0005] Monohydrate

[0006] The Monohydrate processing steps involve: calcining the crudemined trona converting it to crude soda ash, dissolving the crude sodaash in water, clarifying the resulting sodium carbonate liquor to removeinsoluble muds from the solution, filtering the solution, passing thefiltrate to an evaporator circuit (multiple effect evaporator ormechanical vapor recompression evaporator) where water is evaporated andsodium monohydrate crystals are formed and separated from the motherliquor, recycling the mother liquor to the evaporator circuit, andcalcining the monohydrate crystals to soda ash.

[0007] The processes for the production of soda ash and other sodiumcarbonate salts employ crystallization steps that concentrate impuritiesin the mother liquors. Purge streams are required in these processes(Solvay, Sesquicarbonate, Monohydrate, Decahydrate, Sodium Bicarbonate,and processes that recover sodium values from solution mining liquors)to control impurity levels to meet quality requirements. Processes havebeen described in the prior art for recovering the alkali values fromthese various waste streams. Processes that employ the sodiumdecahydrate process must eliminate or significantly reduce the sodiumbicarbonate concentration through decarbonizing using steam stripping,sequential crystallization, or addition of expensive neutralizing agentssuch as caustic soda or lime. Processes that produce more than oneproduct are limited by the demand for the less widely used product:sodium sesquicarbonate, light or medium density soda ash, or sodiumbicarbonate; otherwise, the product in less demand might be furtherprocessed to dense soda ash. These prior art methods are typicallycomplex procedures, involving multiple steps in which various forms ofsodium carbonate are crystallized, and these multiple crystallizationoperations add significantly to the overall economic cost of the sodaash recovery processes.

[0008] Typically, in the manufacture of soda ash, a system of storageponds has been used to accommodate disposal of the insoluble grits andmuds, purge streams, mine water, and other sources of waste watersinherent to the process. These ponds are also used to maintain waterbalance through the process of natural evaporation and can provide asource of cooling water for the evaporator trains. Over the years, asignificant amount of the total mined sodium carbonate has been lostfrom processing as a constituent in the water discharge. As this sodiumcarbonate-bearing water is discharged to the ponds, the waterevaporates, leading to continual concentration of the soda ash in theponds. This soda ash resides in the system through the deposition ofsodium carbonate decahydrate crystals on the pond bottoms, reducing thetotal pond volume. A substantial amount of sodium carbonate decahydrate,which in its crystalline form naturally excludes impurities, resides inthese ponds. This reduction in volume results in two major problems fora facility of substantial commercial size; first, the need to expandexisting or constructing additional waste cells to impound insolubles;and second, inadequate evaporation and hence inadequate cooling of thewater used for cooling purposes.

[0009] Although the insolubles amount to only a small faction of theore, over time these accumulate to sizeable volumes, for example,amounting to over 288,000 tons per year when operating a plant producing2,400,000 tons per year of soda ash. Several methods for disposing ofthese grits, muds, and waste streams have been described in the priorart and involve a process of returning these insolubles to the minetaking advantage of the space created in mined-out areas. However, manyproblems exist because of the presence of water associated with thesolid impurities. The water will drain from the solids over time,creating a messy and hazardous condition that must be confined and thewater collected. For example, the water has been known to dissolve theribs and floor in areas where subsidence has not occurred, compromisingthe mining panel. Recovery of water in subsided panels presents otherproblems. To improve recovery efficiency, the addition of alkalinehydroxides or alkaline earth metals, to concentrations up to 10% of theaqueous slurry solution are described has been employed.

[0010] A complicating factor in dissolving trona deposits is that sodiumcarbonate and sodium bicarbonate that comprise the trona have differentsolubilities and dissolving rates in water. These incongruentsolubilities of sodium carbonate and sodium bicarbonate tend to causebicarbonate “blinding” when employing solution mining techniques.“Blinding” is an occurrence which has long been recognized as a problemby the art in the solution mining industry.

[0011] Solution mining techniques suffer from several disadvantages. Itis apparent that a significant continuing problem associated withsolution mining is the subsequent recovery of sodium carbonate fromrelatively low concentration of carbonate and bicarbonate in thesolution brine. Unless the bicarbonate concentration is reduced,solution mining brines will contain an unacceptable high level of sodiumbicarbonate and other impurities to prevent processing into sodiumcarbonate by the conventional monohydrate process. A major problemexperienced is due to the co-precipitation of sodium sesquicarbonatecrystal during the sodium carbonate monohydrate crystallization whichreduces the quality of the final product. Another difficulty withunderground solution mining is that the requirement of the hightemperatures that are needed to increase the dissolving rate of tronaand yield highly concentrated solutions required as feed to aconventional sodium carbonate monohydrate process. Substantial energy isrequired to heat the solvent sufficiently to off-set heat losses to theearth. Processes that add alkalis such as sodium hydroxide or lime tothe solvent for solution mining reducing the energy requirements whileincreasing the dissolving rate have been demonstrated, but theseneutralizing agents are unfortunately expensive.

[0012] Methods to process such sodium bicarbonate-bearing solutions arehave also been described and comprise a conversion of the alkali valuesto a more desirable crystal or crystals that can then be separated andprocessed to soda ash. When the processes involve two crystallizationsteps such processes yield two different species of sodium carbonatesalts. Such processes describe the addition of expensive neutralizingagents to convert the sodium bicarbonate to sodium carbonate, steam orair stripping to convert the bicarbonate to carbonate and carbon dioxidegas, and even enriching the solution by contact with conventionallymined trona or carbonating the solution with carbon dioxide gas.

[0013] The economic attraction of solution mining is to avoid such costsas sinking new mine shafts, employing miners and equipment underground,and supporting mechanical mining operations. But, these benefits areoffset by the cost of adding neutralizing agents to the slurries pumpedinto the mine and the operational costs and limitations associated withproducing soda ash from such recovered streams.

[0014] A process as herein detailed that reduces the volume of the wastestreams reporting to evaporation ponds would prolong the life of theponds and allow a more economic option for grits and mud disposal thanwould be experienced employing underground disposal methods. Waterbalance would be achieved with the benefit of alkali recovery of thedecahydrate deposits. A process of this kind would enrich the weaksodium carbonate-bearing streams, and if desired, precipitate sodiumbicarbonate from such streams to produce a dilute sodium bicarbonatesolution rich in sodium carbonate and suitable for recovering soda ashemploying sodium decahydrate or sodium monohydrate processes. Such aprocess would also allow selective precipitation of other sodiumcarbonate salts such as sodium sesquicarbonate, sodium bicarbonate, andvarious densities of soda ash.

SUMMARY OF THE INVENTION

[0015] The present invention involves a novel process that employs aproportionate blending of different waste streams to enhance the levelof recovery of sodium values from the waste streams. Crystallizationprocesses employed in the commercial production of sodium carbonatesalts require purge waste streams to exhaust concentrated impuritiesfrom the processes to achieve and maintain final product qualityrequirements. Solids present in the ore feed to the processes aretypically removed in the liquor preparation step and are discharged tosurface evaporation ponds or deposited underground, typically inabandoned mining panels. Trace species such as organics, chloride,silica and sulfate concentrate in the mother liquors separated from thedesired crystals, a portion of which must be purged. The presentinvention deals with a series of processes whereby sodium values arerecovered from these waste streams, and other waste streams such as minewater, evaporation pond supernatant, and other sodium bearing streams,prolonging the life cycle of surface impoundments and avoiding problemsinherent to underground disposal schemes.

BRIEF DESCRIPTION OF THE DRAWING

[0016]FIG. 1 is a flow diagram, which schematically illustrates theprocess of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Specifically, the present invention combines a variety of wastestreams in proper proportions to produce a liquor stream wherein thesodium bicarbonate is reduced so as to suitably feed a sodium carbonatedecahydrate process and recover soda ash. The beneficial results fromthe invention include:

[0018] 1. Reduces operating costs for producing sodium carbonate liquorsin processes such as sodium carbonate monohydrate processes bydisplacing liquors produced directly from mined trona ore and/orsolution mining processes with those produced from recovered sodiumvalues in waste streams;

[0019] 2. Allows for combining various purge streams which are saturatedin the desired salt with waste streams of lesser concentration, dilutingsaid impurities to levels acceptable in recycle streams to theprocessing units; when the combined streams are further concentratedwith sodium carbonate decahydrate, these streams are purified further;

[0020] 3. Provides a sodium carbonate decahydrate process capable ofrecovering pure sodium carbonate crystals from streams too dilute to beefficiently recovered using the sodium carbonate monohydrate process.The sodium carbonate decahydrate crystal formed rejects impurities inthe mother liquor and bonds with 10 molecules of water. The molecularwater is pure, not contaminated in proportion to the concentratedimpurities rejected by the sodium carbonate decahydrate crystal.

[0021] 4. Reduces the final purge stream volume reporting to theevaporation ponds. Rather than purging individualevaporation/crystallization units to waste, those various purge streams,or combinations of said streams with other waste streams, are fed to thedecahydrate unit. The single bittern purge stream from this unit is ofless volume than the combined purge streams of the individualevaporation/crystallization units.

[0022] 5. Extends life cycle of surface evaporation ponds. The purgestream reporting to the ponds is half the concentration and flow aswould report without recovery achieved with the present invention.

[0023] 6. Avoids the costs and hazards of other waste disposal schemessuch as underground disposal.

[0024] 7. The soda ash recovered from waste streams is more economicallyproduced than soda ash produced using mechanical or solution miningmethods.

[0025] 8. Avoids problems associated with sequential crystallization orproduction of more than one salt, especially when the production of thesalt of higher commercial demand is limited by the production of thesecond salt of lower commercial demand.

[0026] It is known that some waste streams generated in sodium carbonateprocesses contain sodium bicarbonate at concentrations that make themunsuitable as direct feed to monohydrate processes. Some typical wastestream compositions are shown below: STREAM Na₂CO₃ NaHCO₃ Na₂SO₄ NaClMine Water 10.6-23.8% 1.4-5.0% 0.06-0.6%  0.5-2.8% Pond Water  6-22%0.6-1.6% 0.4-1.6% 0.2 -0.6% Total Containment  1-12% .03-.5%  .07-1.2%.01 -0.3% Water Other Purge Streams  3-14%  3-13%   1% 1.1% MonohydratePurge 30.1% 1.2% 1.7% 1.0%

[0027] Waste stream flow rate and sodium bicarbonate concentration mustbe considered to determine the most efficient and economical method forintroducing the waste stream. Diluting the sodium bicarbonate withstronger sodium carbonate streams, such as sodium decahydrate crystalscan minimize the cost of steam stripping and neutralization. It isdesirable to increase the sodium carbonate concentration to at least 18%while reducing the sodium bicarbonate concentration to below 3.5%. Sucha combination will produce a feed stream suitable for feeding adecahydrate unit. A suitable monohydrate feed stream can be achieved byconcentrating the sodium carbonate to above about 29% while reducing thesodium bicarbonate concentration to below about 1%. It is desirable toachieve less than about 1.4% sodium sulfate through such combinations sothat the undesirable burkeite (Na₂CO₃.2Na₂SO₄) crystal formation isavoided. In instances where such combination of streams can beconcentrated in sodium carbonate and fed to a sodium bicarbonateprocess, the cost for converting sodium bicarbonate can be avoidedentirely.

[0028] Consideration must be given to the seasonal concentration ofsodium carbonate in streams such as evaporation pond supernatant. Thesupernatant is below the saturation concentration for sodium chloride,sulfate, and bicarbonate, but saturated in sodium carbonate at ambientconditions. The deposited decahydrate solids present in the evaporationponds can be harvested to enrich the sodium carbonate concentration ofthe supernatant, if desired, during the colder months of the year.Having rejected impurities to the supernatant mother liquor whendeposited, these deposits, when harvested, purify the supernant whendissolved, especially when the mother liquor was removed from thesurface of the deposits when the pond water was cold and the sodiumcarbonate concentration the lowest. Such selective removal of motherliquor supernatant to a different evaporation cell during the coldmonths allows “solution mining” of said deposits by a second warm streamwith lower concentrations of sodium bicarbonate, sulfate, and chloride.

[0029] The table below illustrates typical evaporation pond supernatantcompositions during the four seasons of the year and include a typicalanalysis of the deposited decahydrate crystals. Those skilled in the artwill recognize that various options exist for introducing the pondwater, a combination of pond water and harvested decahydrate crystals,or solution mined decahydrate crystals to produce sodium carbonateliquors of sufficient concentration and quality to feedevaporation/crystallization steps and recover various sodium carbonatesalts. Na₂CO₃ NaHCO₃ Na₂SO₄ NaCl Fall Pond Water  8.5% 1.2% 1.3% 0.4%Winter Pond Water   7% 0.7% 0.7% 0.3% Spring Pond Water  13% 0.9% 0.9%0.3% Summer Pond  17% 1.3% 1.0% 0.4% Water Decahydrate  36% 0.1% 0.6%0.1% Deposit

[0030] In accordance with the present invention, combining waste streamsand reducing the sodium bicarbonate concentrations where necessary willconsume the waste streams, utilizing the water present in those streamsand recovering the sodium values therein, thereby reducing significantlythe flow of waste water reporting to the surface evaporation ponds. Withappropriate pond management, the newly created waste purge stream fromthe decahydrate unit will be about 50 percent less than the purge raterequired to maintain equivalent dense soda ash final product quality.The sodium carbonate deposition rate will be about 45 percent less,based on purge rate and sodium carbonate concentration. Decahydrateintroduced into some sodium carbonate streams will increase the waterload when compared to producing from anhydrous soda ash, requiring apurge stream to maintain water balance. Said purge stream will improvethe product quality of the crystal produced when compared to producingthe same salt from anhydrous sodium carbonate and not utilizing a purgestream.

[0031] In the process of the invention the soda-bearing waste water ismixed in appropriate proportions to achieve a liquor with a compositionsuitable for decahydrate unit feed. An optional introduction point inthe treatment process is a steam stripper or a causticizing step whereeither calcium oxide (quick lime), calcium hydroxide (slaked lime),sodium hydroxide (caustic), or other alkali is added to reduce thesodium bicarbonate concentration to below 3.5% when fed to a sodiumdecahydrate unit or to below 1% when fed to a sodium monohydrateprocess. It may be necessary to concentrate the decarbonized liquorprior to feeding the decahydrate or monohydrate processes. The liquorcan be enriched by melting decahydrate formed from the decahydrate step,by introducing decahydrate crystals deposited in the evaporation ponds,by evaporating a portion of the water through a third effect evaporatorbody, cooling tower, or other evaporative step.

[0032] Where a purer sodium carbonate salt is desired, it is beneficialto crystallize said salt from a sodium carbonate decahydrate producedstream. Sodium carbonate decahydrate has the ability to rejectimpurities and bond with 10 molecules of water when crystallized. Thismolecular water is pure, not contaminated in proportion to theconcentrated impurities rejected by the crystal, purifying the liquorstreams into which it is introduced. The sodium carbonate decahydratecrystal is more concentrated in sodium carbonate than saturated sodiumcarbonate liquor is. Purer process liquor with higher sodium carbonateconcentration can be achieved, thus reducing the amount of water to becooled or evaporated in downstream evaporation/crystallization steps.

[0033] The sodium crystals produced from a crystallization/evaporationprocess whereby the feed liquor originated from anhydrous soda ash canbe further purified by introducing sodium carbonate decahydratecrystals. The water imbalance caused by the 10 molecules of waterintroduced with the decahydrate crystal, when purged, will reduce theimpurities otherwise concentrated in the recycle stream. This purgestream, being less concentrated in impurities, is preferably mixed withother waste streams and concentrated appropriately to feed a sodiumcarbonate decahydrate unit. The lower volume, more impure bittern purgestream from the sodium decahydrate unit will be the single source ofwaste water reporting to the evaporation ponds.

[0034] The Process of the invention provides the following benefits:

[0035] 1. Consumes water and concentrates sodium carbonate when varioussodium carbonate bearing waste streams are combined and cooled toprecipitate sodium carbonate decahydrate.

[0036] 2. Purifies the liquor streams into which the formed decahydratecrystals are melted and dissolved. The ten molecules of pure water boundto the sodium carbonate in the decahydrate crystal are pure water, notcontaminated in proportion to the concentrated impurities rejected intothe mother liquor by the decahydrate crystal.

[0037] 3. Provides a concentrated source of sodium carbonate(decahydrate is 37% sodium carbonate compared to 31% available in aconcentrated liquor), purer than sodium carbonate liquors produced frommine ore, solution mined liquor, and other sodium carbonate bearingstreams, to concentrate other streams of less sodium carbonate valueappropriately to feed an evaporation/crystallization step to recoversodium salts, such as with a sodium monohydrate process.

[0038] 4. Sodium carbonate from waste streams fed to evaporation pondsis recovered by “solution mining” sodium carbonate decahydrate depositedoffering a substantial economy over the life of the sodium carbonateplant.

[0039] 5. Crystallization processes used to produce sodium salts requirepurge streams to achieve and maintain final product quality. Theimpurities resident in the feed liquors are concentrated to a motherliquor during the crystallization step. Steps involvingevaporation/crystallization concentrate said impurities further. Theinvention permits combining various purge streams which are saturated inthe desired salt with waste streams of lesser concentration, dilutingsaid impurities to levels acceptable in recycle streams to theprocessing units. When said combined streams are further concentratedwith sodium carbonate decahydrate, these streams are purified further.

[0040] 6. The invention provides a system for combining the variouspurge streams and adjusting the sodium carbonate concentrationappropriately to feed the evaporation/crystallization steps ofdownstream processes such that the bittern purge stream of the sodiumcarbonate decahydrate crystallizer is of sufficient volume to allow puredownstream crystals to be formed, yet is of less volume than that of thecombined individual downstream evaporation/crystallization purgestreams.

[0041] 7. The liquor streams produced through combination andconcentration of existing waste streams and available waste depositsprovides a less expensive source of sodium carbonate feedingevaporation/crystallization steps than is otherwise achieved usingsodium carbonate recovered through conventional mining techniques,solution mining techniques, and other techniques known to the art.

[0042] 8. Extends the life cycle of surface evaporation ponds. The purgestream reporting to the pond is half the concentration and flow as wouldreport without recovery achieved with the present invention.

[0043] 9. Avoids the costs and hazards of other waste disposal/recoveryschemes such as underground disposal and solution mining.

[0044] The invention is described in greater detail by reference to theaccompanying flow diagram: Stream 1, low in sodium carbonate/sodiumbicarbonate concentration, is heated using waste heat (stream 2) fromsources such as a condenser in a triple effect evaporator or waste heatfrom a steam stripper, in heat exchanger 10 and sent to evaporation pond20, where it dissolves previously deposited sodium carbonate decahydratecrystals. The enriched stream, 21, is combined with other sodiumbicarbonate containing flows (stream 22) obtained from other processesand is further heated in heat exchanger 30, also with waste heat (stream36). The resulting stream, 31, can be recycled back through stream 32 tothe previous steps in order to increase its sodium values and/ortemperature. If the sodium values and temperature are deemed acceptable,then stream 31 can be processed through causticizer 40 or steam stripper50 (stream 33).

[0045] After stream 31 has reached 120° F.-140° F., it is fed tocausticizer 40, where the addition of lime (stream 42) and/or sodiumhydroxide (stream 43) converts bicarbonate values to carbonate valuesand produces calcium carbonate (CaCO₃) as a byproduct. Stream 41,leaving causticizer 40, has less than 3.5% sodium bicarbonate contentand it is directed to separator 60, where the calcium carbonatebyproduct is removed from the process. Stream 61, now low on sodiumbicarbonate values, is directed to mix tank 70, where it is blended withother streams for further treatment by way of a decahydrate process orit can be sent directly to a monohydrate process.

[0046] Mix tank 70 blends the recovery stream low on sodium bicarbonate(61) with mine water (73), monohydrate crystallizer purge (122),concentrated waste streams (72), and other waste streams (74). Thisblend is then fed to decahydrate crystallizer 80, where sodiumdecahydrate crystals are produced. These crystals are then separatedfrom the mother liquor in centrifuge 90. A portion of this mother liquor(stream 92) can be recycled (stream 94) and the remainder can be sent tothe ponds (stream 93). Purge stream 93 is adjusted to maintain theconcentration of Na₂SO₄ present in the mother liquor below 1.4%. Thedecahydrate crystals can be sent to concentrator 100 (via streams 91,96) or they can be sent to monohydrate feed tank 110 (via streams 91,95).

[0047] Concentrator 100 receives decahydrate crystals and stream 51 fromsteam stripper 50. From here this stream can be directed towards otherprocesses (stream 102) or to monohydrate feed tank 110. Feed tank 110feeds liquor to monohydrate crystallizer 120, where sodium monohydratecrystals are produced. These crystals are then separated from the motherliquor in centrifuge 130. The separated crystals are dried to anhydroussodium carbonate crystals in dryer 140 and exit the process as finishedproduct via stream 141. The mother liquor (stream 132) is recycled backto feed tank 110 for further recovery of sodium values. Purge stream 122is required in order to maintain the Na₂SO₄ levels below 1.4% in themonohydrate crystallizer mother liquor.

[0048] In addition, once stream 31 reaches desired levels of temperatureand sodium concentration, it is possible to send this stream to steamstripper 50, where sodium bicarbonate is transformed to sodium carbonateby the introduction of steam (stream 55). CO₂ is the byproduct of thisoperation (stream 53) and it can be disposed or recovered. Stream 51,now having less than 1% sodium bicarbonate, can be further treated byway of a decahydrate process (stream 52) and/or a monohydrate process.

[0049] It is thus seen that in the process of the present invention,wherein soda values are recovered from various waste streams and othersodium carbonate bearing streams, impurities are rejected to a bitternspond at a rate less than that required when not practicing the instantinvention. The bitterns purge rate is determined based on the finalproduct quality requirements for individual unit operations producingsodium carbonate decahydrate, sodium carbonate monohydrate, and othersodium salts. The individual unit operation purge streams are combinedappropriately to produce a liquor feed stream comprised of the wastedsodium value that is then recovered through sodium decahydratecrystallization. The crystal formed adequately rejects the impuritiesresident in pregnant liquors from which it was produced, thispurification step consuming water and sodium carbonate that can enrichother sodium bearing streams, or feed directly downstreamevaporation/crystallization processes such as sodium carbonatemonohydrate processes.

[0050] The process of the invention utilizing the following advantageousfunctional steps:

[0051] 1. Sodium bicarbonate bearing waste streams are suitablydecarbonized to reduce the sodium bicarbonate concentration prior tosodium decahydrate or sodium monohydrate crystallization.

[0052] 2. The sodium bicarbonate concentration in the bicarbonatebearing stream is reduced using any suitable known method, such asreacting said sodium bicarbonate with a neutralizing agent such ascalcium oxide, calcium hydroxide, sodium hydroxide or other alkalis.Steam and air stripping can be employed. Diluting the sodium bicarbonateconcentration using a variety of other streams or by the addition ofsodium carbonate decahydrate will reduce the sodium bicarbonateconcentration.

[0053] 3. The calcium carbonate formed during the causticizing step isseparated from the sodium carbonate solution and can be disposed of orfurther processed to form a calcium chloride solution with the reactionof hydrochloric acid.

[0054] 4. The liquor produced by combining the various sodium carbonatebearing streams is concentrated to an appropriate sodiumcarbonate/sodium bicarbonate concentration. This is accomplished usingpurge streams from the sodium carbonate monohydrate process ordecahydrate crystals from the decahydrate process.

[0055] 5. The concentration of sodium bicarbonate can be maintainedbelow 3.5% and is maintained preferably below 1.5% to avoidco-precipitation with sodium decahydrate in the evaporative coolingstep.

[0056] 6. The concentration of sodium sulfate can be below 1.4% to avoidburkeite crystal co-precipitation with sodium decahydrate in theevaporative cooling step.

[0057] 7. The concentration adjusted liquor stream undergoesevaporation/crystallization to form the specific desired sodiumcarbonate salts, such as sodium carbonate decahydrate and sodiumcarbonate monohydrate.

[0058] 8. The sodium carbonate decahydrate crystals can be used tofurther concentrate more dilute sodium carbonate bearing streams, or canbe further processed to form light or medium density anhydrous soda ash.

[0059] 9. Sodium decahydrate crystals can replace anhydrous sodiumcarbonate or sodium carbonate monohydrate as process liquor make-up. Thedecahydrate introduces 10 molecules of pure water that may result inwater imbalance. Purer sodium crystals are formed when such excess waterand the impurities concentrated in downstreamcrystallization/evaporation steps are therefore purged at a higher rate.

[0060] 10. Various separate purge streams are combined and adjusted insodium carbonate/bicarbonate/sulfate concentration appropriately to feeda sodium carbonate decahydrate unit. The bittern purge stream of saiddecahydrate unit is of lower volume and higher impurity concentrationthan the combination of separate various purge streams. This purge rateis determined by the accumulation of sodium bicarbonate and/or sodiumsulfate that when concentrated co-precipitate in the decahydratecrystallization step as described previously.

[0061] 11. The bittern purge rate is estimated to be at least 50 percentless in volume and 45 percent less concentrated in sodium carbonatevalue than would be present if the individual unitevaporation/crystallization unit purge streams were deposited in theevaporation ponds and sodium carbonate decahydrate crystals notrecovered.

[0062] 12. The life cycle of the surface evaporation ponds will be aboutdoubled because of the lower flow and concentration of the bittern purgestream. The costs and hazards of underground disposal are avoided.

[0063] 13. The soda ash recovered from waste streams is moreeconomically produced than soda ash produced using mechanical mining orsolution mining methods.

[0064] Although the invention has been described in terms of particularembodiments, variations to the details described herein can be made andsubstitutes for known process aides known to those skilled in the artcan be used without departing from the invention. Thus, the invention isnot meant to be limited to the details described herein, but only by thescope of the appended claims.

What is claimed is:
 1. A process for recovering a pure soda ash productfrom impure and relatively depleted sodium carbonate bearing streamscomprising: a) combining a plurality of sodium carbonate bearing streamsin a proportion to suitably feed a sodium carbonate decahydrate process;b) neutralizing and thereby reducing the bicarbonate concentration ofsaid combined streams by a means selected from treatment with (i) aneutralizing agent, (ii) decarbonizing, (iii) diluting with the additionof a more highly concentrated sodium carbonate bearing stream, and (iv)combination of neutralizing, decarbonizing, and diluting; c)crystallizing sodium salts from said combination of streams to formsodium carbonate decahydrate; d) sufficiently purging and recyclingaccumulated impurities from steps a through c; e) utilizing thedecahydrate from step d to concentrate a less concentrated sodium saltblend; f) crystallizing sodium carbonate product having a desired levelof sodium carbonate from the concentrated product of step c; g) wastingpurge stream from step d to surface evaporation ponds to avoid the costsand hazards associated with underground disposal methods.
 2. The processof claim 1 wherein the purge steam from step d is utilized to effect aprocessing step selected from to (i) concentrating a less concentratedsodium carbonate stream, (ii) feeding the sodium decahydrate unit, and(iii) both concentrating a less concentrated sodium carbonate stream andfeeding a sodium carbonate decahydrate unit.
 3. A process for producingcalcium chloride by withdrawing calcium carbonate from theneutralization by-product of step b of claim
 1. 4. A process accordingto claim 1 wherein the lesser sodium carbonate bearing waste streamsinclude mine water, pond water, other sodium carbonate bearing streamssuch as containment basins used to comply with environmental liquiddischarge permits, and other process waste streams with concentrationsless than about 18% sodium carbonate.
 5. A process according to claim 1wherein the higher concentrated sodium bearing waste streams includepond water, enriched warm water introduced to impounded sodiumdecahydrate deposits with the purpose of enriching said warm water insodium concentration by melting and dissolving said deposits, streamsenriched in sodium carbonate concentration by mechanically mining saidimpounded sodium decahydrate deposits, sodium carbonate monohydratepurge streams and other sodium carbonate evaporator/crystallizer purgestreams with concentrations greater than about 18% sodium carbonate. 6.A process according to claim 1 wherein separate or combined wastestreams are enriched in sodium carbonate concentration to crystallizethe specific sodium carbonate salt species desired that includes: a)combining streams of lesser sodium carbonate concentration with streamsof higher sodium carbonate concentrations; b) enriching streams oflesser sodium carbonate concentration with decahydrate crystals; c)evaporating water from streams of lesser or greater sodium carbonateconcentration using such methods as the third effect of a triple effectcrystallizer train, cooling towers, evaporator cooler, air cooled sprayevaporator/crystallizer, or other evaporation methods known in the art;d) and appropriate combination of the above.
 7. A process according toclaim 1 wherein separate or combined waste streams are enriched insodium carbonate concentration to crystallize the specific sodiumcarbonate salt species desired that includes: a) combining streams oflesser sodium carbonate concentration with streams of higher sodiumcarbonate concentrations; b) enriching streams of lesser sodiumcarbonate concentration with decahydrate crystals; c) evaporating waterfrom streams of lesser or greater sodium carbonate concentration usingsuch methods as the third effect of a triple effect crystallizer train,cooling towers, evaporator cooler, air cooled sprayevaporator/crystallizer, or other evaporation methods known in the art;d) and appropriate combination of the above.
 8. A process according toclaim 1 wherein separate or combined waste streams are depleted insodium bicarbonate concentration as appropriate to crystallize sodiumcarbonate decahydrate used as described in the instant patent, or usedto concentrate sodium carbonate in streams feeding other sodiumcarbonate salt processes.
 9. A process according to claim 2 whereinseparate or combined waste streams are depleted in sodium bicarbonateconcentration as appropriate to crystallize the specific sodiumcarbonate salt species desired that includes dense sodium carbonate andsodium decahydrate used as described in the instant patent, or used inthe production of medium or light density sodium carbonate.
 10. Aprocess that extends the life cycle of surface evaporation ponds whereinthe purge stream in step b of claim 1 is received by about one-half theconcentration and flow of the combined purge streams from monohydrateand other known sodium salt evaporation/crystallization processes.
 11. Aprocess that substantially reduces the hazards of accumulated wastestream disposals comprising treating the stream according to the processof claim 1.